We have people in different countries working on the same projects. It would be nice if each person/resource could have an individual set of holidays or vacations assigned to them, and then to have these days/periods marked in a special color on the task bars to which the resources have been assigned (and on the availability-chart bars for those resources).

In this way those resources' non-availability would be clearly signaled on the chart, without any need for automatic lengthening/shortening of task duration when such a resource is assigned or removed from a task.

To summarize: The company-wide holidays would continue to work as before, by removing available working days from the Gantt calendar; whereas resource-specific holidays would merely be signaled by specially coloring a portion of the task bar to which the resource has been assigned. (If multiple resources have been assigned to a task and only one of those resources has custom holidays during the task period, the special coloring could appear e.g. in a lighter hue).